Wind turbines operate by transferring the motion of wind into the rotation of aerodynamic surfaces (e.g., airfoil blades) around a horizontal or vertical axis. The aerodynamic surfaces are connected to an electromechanical transducer (e.g., generator) that converts the mechanical kinetic energy of rotation into electrical energy. In order to avoid destructive aerodynamic interference between adjacent wind turbines that are arranged in an array on a wind farm, the turbines are typically spaced at least two rotor diameters apart both laterally and in the windward direction. This requirement results in significant land use by wind farms.
U.S. Pat. No. 6,784,566, incorporated herein by reference in its entirety, describes a method to achieve constructive aerodynamic interference between a pair of counter-rotating vertical-axis wind turbines (henceforth VAWTs).